HTML
<img src=//ph.artsinn.de alt>
<div>
<p>A short line of text, for better interpretation.</p>
<p>Followed by another line, filled with letter and words</p>
</div>
<img src=//ph.artsinn.de/240x160/666?text=scroll%20down alt id=obj>
<img src=//ph.artsinn.de alt>
<img src=//ph.artsinn.de alt>
<img src=//ph.artsinn.de alt>
<img src=//ph.artsinn.de alt>
CSS
/* not needed */
html,body{height:200%}
img {display:block}
JS
$(function() {
$window = $(window),
$sidebar = $("#obj"),
sidebarTop = $sidebar.position().top,
$sidebar.css('position', 'fixed');
$window.scroll(function(e) {
scrollTop = $window.scrollTop(),
topPosition = Math.max(0, sidebarTop - scrollTop),
$sidebar.css('top', topPosition);
});
});
デモ